Integrated circuit metallization using a titanium/alminum alloy

ABSTRACT

An integrated circuit metallization structure using a titanium/aluminum alloy, and a method to generate such a structure, provide reduced leakage current by allowing mobile impurities such as water, oxygen, and hydrogen to passivate structural defects in the silicon layer of the IC. The titanium layer of the structure is at least partially alloyed with the aluminum layer, thereby restricting the ability of the titanium to getter the mobile impurities within the various layers of the IC. Despite the alloying of the titanium and aluminum, the metallization structure exhibits the superior contact resistance and electromigration properties associated with titanium.

[0001] This application is a division of application Ser. No.09/698,459, filed Oct. 27, 2000, hereby incorporated herein byreference.

BACKGROUND OF THE INVENTION

[0002] In the field of integrated circuit (IC) technology, the area ofinterconnect metallization has received a great deal of attention fromresearchers and developers over the years due to its importance withinthe overall IC physical structure. Although the metallization layers ofan IC are used primarily to connect various circuit elements within theIC silicon, the metallization also has a direct impact on attainabledevice geometry, defect density, and leakage current, making ICmetallization a critical area of study.

[0003] As an example, the metallization of an IC influences the densityof mobile impurities within the silicon and dielectric layers thatsurround the interconnections. For example, researchers have identifiedseveral metallic substances that are useful as “gettering” materials,which are capable of trapping various mobile impurities in the siliconand dielectric layers of an IC. Generally speaking, gettering is adesirable process within an integrated circuit, mitigating the effectsof mobile impurities that are commonly introduced during the ICfabrication process. The effects of such impurities include decreaseddevice performance, reliability, and processing yield, among others. Ingeneral, gettering reduces these effects by restricting the movement ofthe mobile impurities, thus improving overall IC performance.

[0004] However, some cases exist in which too much gettering ispossible; in other words, the presence of some particular types ofmobile impurities, in moderate quantities, are actually beneficial tothe performance of an integrated circuit. For example, it has been shownin the art that titanium is beneficial as a component of ICmetallization for several reasons. As a gettering substance, titaniumtraps water, hydrogen, and oxygen, thus allowing these substances to beabsorbed readily from within the silicon and dielectric layers of an IC.The gettering properties of titanium are discussed, for example, inMarwick, A. D., et al., “Hydrogen redistribution and gettering inAlCu/Ti thin films” in Journal of Applied Physics, Vol. 69, No. 11, June1, 1991, pp. 7921-23, and Yoshimaru, M., et al., “Deoxidation of WaterDesorbed from APCVD TEOS-O₃ SiO₂ by Titanium Cap Layer” in Proceedingsof the 1995 IEEE International Reliability Physics Symposium, pp.359-64. Additionally, titanium exhibits low contact resistance and helpsimprove electromigration properties in aluminum. In some circumstances,unfortunately, the gettering effect produced by titanium causes too manywater molecules, and the hydrogen and oxygen that combine to form thewater, to be absorbed within the titanium. For instance, water and itsconstituent elements are useful under certain conditions for passivatingstructural defects within silicon by bonding with the defect sites, thuscausing the IC to function more efficiently. In that case, substantialgettering of hydrogen and oxygen may actually be a detriment to theperformance characteristics of the IC, resulting in increased leakagecurrent and other impediments to optimal device performance.

[0005] Therefore, in many cases it would be advantageous to construct ametallization structure that utilizes the positive qualities oftitanium, such as mitigation of electromigration effects and low contactresistance, while at the same time restricting the gettering effect ofthe titanium so that water, hydrogen, and oxygen will be available topassivate structural defects in the silicon layer of the device.

SUMMARY OF THE INVENTION

[0006] Specific embodiments according to the present invention, to bedescribed herein, provide a useful way for titanium to be utilized in ametallization structure of an integrated circuit. The proposed structuretakes advantage of the desirable electromigration and contact resistanceproperties of the titanium, while at the same time limiting itsgettering capabilities. Consequently, a sufficient amount of mobileimpurities, such as water (and the hydrogen and oxygen it comprises),are then available to passivate structural defects in the silicon layerof the IC.

[0007] A method embodiment of the invention begins with the depositionof a layer of titanium onto a preexisting layer of an integrated circuitduring fabrication of the IC. A layer of aluminum is then depositeddirectly onto the titanium layer. During heating of the IC that normallyoccurs in subsequent fabrication process steps, at least a portion ofthe titanium alloys with the aluminum layer. The advantage of alloyingthe aluminum and titanium is that the gettering capacity of the titaniumis restricted, thus allowing mobile impurities such as water, hydrogen,and oxygen to be available to passivate structural defects with thesilicon of the IC. At the same time, titanium still helps set thestructural texture of the aluminum layer, providing low contactresistance and improved electromigration properties.

[0008] Another embodiment of the invention describes a metallizationstructure as it resides on a preexisting layer of the IC after thefabrication of the IC. A layer of titanium resides on the preexistinglayer. A layer of aluminum then resides on top of the titanium, with thetitanium layer being at least partially alloyed with the aluminum layer.Consequently, the portion of titanium that is alloyed with the aluminumis no longer available as a gettering species, while still providing thedesirable electromigration and contact resistance properties normallyassociated with titanium.

[0009] Other aspects and advantages of the invention will becomeapparent from the following detailed description, taken in conjunctionwith the accompanying drawings, illustrating by way of example theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is an idealized cross-sectional view of an integratedcircuit metallization structure according to an embodiment of theinvention after deposition of the metal layers.

[0011]FIG. 2 is an idealized cross-sectional view of an integratedcircuit metallization structure according to an embodiment of theinvention after oxide and passivation layer deposition.

[0012]FIG. 3 is an idealized cross-sectional view of an integratedcircuit metallization structure according to an embodiment of theinvention after final annealing.

[0013]FIG. 4 is a flow chart of a method of fabricating a metallizationstructure according to an embodiment of the invention.

DETAILED DESCRIPTION

[0014] A cross-sectional view of an integrated circuit duringfabrication immediately after metal layer deposition is shown in FIG. 1.The integrated circuit may be of any type contemplated, including, butnot limited to, an application-specific integrated circuit (ASIC), amicroprocessor, an analog IC, an optoelectronic IC, and so on. For theembodiment of FIG. 1, the structural basis for all layers deposited onthe integrated circuit is a silicon layer 10, which in the general caseis a standard silicon wafer. Silicon layer 10 has a top silicon surface5, upon which other layers of the IC are deposited. As is widely known,semiconductor devices (not shown) that make up the various electronicelements of the integrated circuit reside primarily in or on siliconlayer 10. Also shown in FIG. 1 are structural defects 7, which reside attop silicon surface 5 of silicon layer 10, as well as within siliconlayer 10. Structural defects 7 result from normal wafer fabricationprocesses. Atop silicon layer 10 is deposited a first dielectric layer20, which in most cases is a layer of silicon dioxide. A metallizationstructure 30 is then deposited over first dielectric layer 20, whichprovides electrical insulation between metallization structure 30 andsilicon layer 10. Metallization structure 30 is actually made up of manywires, or “traces,” of which one is shown in FIG. 1 for the sake ofsimplicity. Electrical contacts (also not shown) made of metal exist atpredefined places within first dielectric layer 20 to allowinterconnection of the semiconductor circuit components within siliconlayer 10 by way of metallization structure 30.

[0015] According to the embodiment of FIG. 1, metallization structure 30is made up of three separate metal layers at the time of deposition. Atitanium layer 32 is deposited first on dielectric layer 20, followed byan aluminum layer 34, and finally by a titanium-nitride layer 36.Titanium-nitride layer 36 is used in this embodiment as a top claddinglayer to provide some electromigration protection and to serve as ananit-reflection coating to aid in patterning. Other embodiments may notuse a top cladding layer or layers at all. Still others will usealternate materials, including, but not limited to, titanium-tungsten,titanium-tungsten-nitride, tungsten, tungsten-nitride, tantalum,tantalum-nitride, and molybdenum. The existence of titanium layer 32 asdeposited on dielectric layer 20 provides for improved contactresistance when used to connect circuit components of silicon layer 10with metallization structure 30 when compared with the contactresistance of other metallic substances, such as titanium-nitride andtitanium-tungsten. Also, titanium layer 32 improves the electromigrationproperties of aluminum layer 34. In other words, aluminum layer 34develops fewer cracks in its lattice structure as a result of electricalcurrent through the metal. Such cracks ultimately can cause open circuitconnections within the metallization layers of an IC.

[0016] As stated earlier, titanium also acts as a gettering species bybinding with mobile impurities within the integrated circuit, therebyrendering the impurities ineffective as charge carriers. While thiseffect is sometimes beneficial within an IC, there are specificinstances when gettering is not desirable. For example, the impuritiesgettered by titanium include water and its constituent elements,hydrogen and oxygen; in other words, titanium removes water, hydrogen,and oxygen from the surrounding areas of the IC. However, water,hydrogen, and oxygen, which are normally present during the ICfabrication process, are useful agents in passivating structural defects7. The passivating agents bind with the dangling bonds of structuraldefects 7, thereby inhibiting the ability of the dangling bonds toprovide a path for leakage current within silicon layer 10. Leakagecurrent generally occurs in semiconductor circuits in places when littleor no current is expected or desired, such as into the gate of a MOSFET,or through the channel of a MOSFET when biased in the “OFF” state.

[0017] Another example of unwanted leakage current is the “dark current”associated with a photodiode within an optoelectronic IC. In that case,the photodiode should generate a miniscule amount of current under darkconditions. However, the presence of defects on the top surface of thesilicon will cause a higher-than-expected dark current, which wouldindicate the presence of light incident on the photodiode when, in fact,there is none. In situations such as these, passivation of structuraldefects 7 would improve the functionality of the circuit significantly.Metalization structure 30, according to embodiments of the presentinvention, provides this ability.

[0018]FIG. 2 displays the cross-section of the same portion of theintegrated circuit as shown in FIG. 1, but after a second dielectriclayer 40 and a passivation layer 50 have been deposited. Seconddielectric layer 40, which typically is silicon dioxide, serves toelectrically isolate portions of metallization structure 30 from anysubsequent layers deposited on the IC. Afterwards, passivation layer 50is added to provide overall protection to the IC during the remainder ofthe IC fabrication process. Passivation layer 50 also acts as a barrierthat traps water, hydrogen, and oxygen within the IC. In the embodimentpresented in FIG. 1, FIG. 2, and FIG. 3, only one metallization layer isimplemented. However, two or more layers are formed in the typicalintegrated circuit. In those embodiments, metallization structure wouldbe utilized for each metallization layer with the IC.

[0019] During the deposition and other processing for second dielectriclayer 40 and passivation layer 50, metallization structure 30 is heatedsufficiently to cause at least partial alloying of titanium layer 32(from FIG. 1) and aluminum layer 34, thereby creating titanium-aluminumalloy layer 38.

[0020] The embodiment of FIG. 2 exemplifies the case in which theentirety of titanium layer 32 is alloyed into aluminum layer 34, therebynot leaving any “pure” titanium remaining in metallization structure 30.Such a condition is desirable in the case of the optoelectronic ICmentioned above. The presence of any pure titanium in metallizationstructure 30 would allow at least some partial gettering of water andthe associated hydrogen and oxygen. As a result, the number of mobileimpurities available to passivate the defects at top silicon surface 5of silicon layer 10 would be reduced, and the dark current of thephotodiodes would increase. According to tests performed usingmetallization structure 30 in an optoelectronic IC, titanium layer 32with a thickness of less than or equal to approximately 200 angstromscan be completed alloyed with aluminum layer 34. However, othermanufacturing processes may allow thicknesses of greater than 200angstroms for titanium layer 32 to be completely alloyed with aluminumlayer 34, resulting in the same passivation effects noted for theabove-mentioned optoelectronic IC.

[0021] In other embodiments, more gettering may be desirable; underthose circumstances, titanium layer 32 may be made thicker so that onlypartial alloying of titanium layer 32 with aluminum layer 34 occurs,thus leaving a portion of titanium layer 32 to remain withinmetallization structure 30. In such a case, gettering of water and itsconstituent elements is gained at the cost of reduced passivation ofstructural defects 7 at top silicon surface 5 of silicon layer 10, aswell as within silicon layer 10.

[0022]FIG. 3 is an idealized cross-sectional view of the integratedcircuit after a final annealing step has been completed. During finalanneal, the IC is heated to a high temperature, with that temperaturebeing maintained for an extended period of time. The final annealingallows the water, hydrogen, and oxygen within first dielectric layer 20and second dielectric layer 40 to diffuse rapidly to silicon layer 10,where structural defects 7 (of FIG. 2) reside. The hydrogen and oxygenthen come in contact with structural defects 7, bonding with theirassociated dangling bonds, thereby converting structural defects 7 topassivated structural defects 8, which do not contribute to thedeleterious effects earlier attributed to structural defects 7. As aresult, effects such as leakage current (or, for optoelectronic ICs,dark current) are reduced since passivated structural defects 8 do notpossess the dangling chemical bonds necessary to provide a conductivepath for leakage current. Tests conducted for metallization structure 30utilized a final annealing phase at 400 degrees C. for about 45 minutes,which was sufficient to allow substantial defect passivation. This finalannealing also served to ensure complete alloying of a 200 angstromtitanium layer in the case that the titanium had not been completelyalloyed with the aluminum by that point in the manufacturing process.Other combinations of temperature and heating time period may beutilized as long as the water, hydrogen, and oxygen of first and seconddielectric layers 20 and 40 are able to diffuse through those layerssufficiently to bond with structural defects 7.

[0023] Another embodiment of the invention is a method of constructing ametallization structure containing titanium that provides superiorcontact resistance and electromigration properties while at the sametime allowing water, hydrogen, and oxygen trapped in the IC to passivatedefects on the surface of a silicon wafer. The steps, shown in FIG. 4,begin with the deposition of a layer of titanium onto a preexistinglayer during the fabrication of an IC (step 400). A layer of aluminum isthen deposited over the titanium layer (step 410). In this embodiment, alayer of titanium-nitride is then deposited over the layer of aluminum(step 420). In other embodiments, the titanium-nitride layer may beomitted. Also, other materials, such as those mentioned earlier, may beused as a top cladding layer in place of the titanium-nitride layer.Thereafter, a heating process is applied to the Ti/Al/TiN metallizationstack (step 430), resulting in the titanium layer at least partiallyalloying with the aluminum layer. The heating step may be a finalanneal, which is a typical process employed in IC fabrication to helprestore previous silicon crystal damage. However, in other embodiments,other heating processes may be utilized to allow the alloying of thetitanium and aluminum.

[0024] The thickness of the titanium layer is determined by the needs ofthe particular application. However, if restriction of the getteringproperties of titanium is of primary concern, such as in the case ofoptoelectronic ICs, a thin titanium layer is warranted so that no puretitanium is present in the metallization stack after the heating process(step 430) is completed.

[0025] From the foregoing, it will be apparent that the inventionprovides a metallization structure that takes advantage of theelectromigration and contact resistance properties of titanium, whilesimultaneously restricting the gettering properties of that metal. As aresult, the water, hydrogen, and oxygen not gettered by the titanium areallowed to passivate structural defects that would otherwise cause anunwanted increase of leakage current in the IC.

What is claimed is:
 1. A method of constructing a metallizationstructure on a preexisting dielectric layer of an integrated circuitduring fabrication of the integrated circuit, the method comprising thesteps of: depositing a layer of titanium onto the preexisting dielectriclayer of the integrated circuit; depositing a layer of aluminum onto thelayer of titanium; heating the integrated circuit sufficiently to causethe layer of titanium to become at least partially alloyed with thelayer of aluminum; and further heating the integrated circuit at 400degrees C. for about 45 minutes so that impurities from the dielectriclayer have passivated structural defects within a silicon layer of theintegrated circuit.
 2. The method of claim 1, wherein the thickness ofthe layer of titanium as deposited is limited so that the layer oftitanium will completely alloy with the layer of aluminum as a result ofthe heating of the integrated circuit.
 3. The method of claim 1, whereinthe thickness of the layer of titanium as deposited is less than orequal to 200 angstroms thick.
 4. The method of claim 1, furthercomprising the step of depositing a layer of titanium-nitride onto thelayer of aluminum.